Conventionally, in a transmission having a clutch with N speed stages, a control valve 21 is constructed, as shown in FIGS. 4 and 5, such that the N number of spools 22 and electromagnetic valves (not shown) are arranged so that each is connected to a corresponding clutch. An electrical signal is transmitted to the electromagnetic valves, and the spools 22 are switched by pilot pressure, thus changing the speed stages.
More specifically, when an electromagnetic valve is turned ON, the pilot pressure acts on the spool 22 against the force of a spring 23 so as to urge the spool to the right, as illustrated in FIG. 4. The hydraulic pressure from a pump (not shown) passes through the control valve 21 from a pump port 24 to a clutch port 25 and is transferred to a clutch (not shown), thus turning the clutch ON.
On the other hand, when an electromagnetic valve is turned OFF, the pilot pressure is reduced to nothing, and thus the spool 22 is pushed to the left by the force of the spring 23, as shown in FIG. 5. The hydraulic pressure of the clutch (not shown) passes through the control valve 21 from the clutch port 25 to a drain port 26 and is drained to an oil tank (not shown), thus turning the clutch OFF. It should be noted that a throttle 27 is arranged so that it communicates with the oil tank when an electromagnetic valve is OFF.
However, in such a conventional construction, when a signal is transmitted from the controller for switching an electromagnetic valve, the controller is not capable of detecting whether or not the spool is switched in response to the signal, thus causing the following problems.
(1) For instance, in a transmission including a low stage clutch and a second stage clutch, although an OFF signal is transmitted to the low stage electromagnetic valve and an ON signal is transmitted to the second stage electromagnetic valve for switching the transmission from low to second, the low stage spool is not always switched.
The reason can be, for example, that the spool or the electromagnetic valve is blocked with dust, or is stuck. In this case, the low stage clutch and the second stage clutch are both engaged, and thus the transmission is damaged.
(2) For example, although an ON signal is transmitted to the low stage electromagnetic valve, the spool is not operated due to a failure of the electromagnetic valve, and the low stage clutch is not turned ON. Even in this case, since the controller is not capable of recognizing an abnormality of the electromagnetic valve, it is necessary to manually operate a spool (hereinafter referred to as an emergency escape spool), which is capable of turning ON and OFF without employing the electromagnetic valve, when the failure is detected. It also takes time to carry out the emergency escape.